1. Field of the Invention
The present invention relates to light emitting devices, and in particular, to a white light-emitting device capable of emitting white light with color temperature tunability.
2. Description of the Related Art
White light-emitting diodes (LEDs) are point light sources that are packaged as a matrix LED for illumination. White light is produced by combining at least two chromatic lights with different wavelengths, such as a blue and yellow light of complementary color or a blue, green and red light.
There are three types of white LEDs presently available. One type uses a white light LED module composed of red, blue and green LEDs, with high luminous efficacy, high color rendering, and real-time tunable color temperature. However, tri-color LED (red, blue and green LED) with different epitaxial materials exhibit different electrical and aging properties, resulting in high cost and complicated driving circuit design for implementation. Also, there is an issue of non-uniform spatial distribution of white light for mixing trichromatic light due to the placement of tri-color LEDs.
Another type of white LED uses a blue LED to excite yellow phosphors to produce white light. The blue LED is coated by an optical resin mixed with yellow phosphors. The blue LED emits blue light with a wavelength of 400-530 nm. The yellow phosphors are excited by the blue light emitted from the blue LED to produce yellow light, and a portion of blue light mixing with the generated yellow light to form the white light with complementary colors. However, the method wherein the white LED uses the blue LED to excite the yellow phosphors has some drawbacks. First, the large amount of transmitted blue light power result in high color temperatures and non-uniform white light distribution. Second, the wavelength of blue light shifts with the LED junction temperature rises, resulting in changes of chromaticity coordinates of the emitted white light. Third, due to the predetermined admixing ratio between yellow phosphor and optical resin of the phosphor resin layer, the color temperature of the white light mixing blue and yellow light is fixed and with limited color temperature tunability.
Still another type of white LED uses ultraviolet (UV) or purple LED to excite multi-color phosphors mixed in transparent optical resin with a specific ratio between phosphors. The UV or purple LED emits UV light or purple light with a wavelength of 320-400 nm. The multi-color phosphors are excited by the UV light or purple light emitted from the UV or purple LED to produce white light. This method has uniform white light distribution and simple driving circuit design for implementation.
However, the method wherein the white LED uses the UV or purple LED to excite the multi-color phosphor has some drawbacks. First, there is a leakage of the purple light or UV light, and this limits its applications. Second, the fixed color temperature of the emitted white light results from the predetermined phosphors in resin ratio, and can not be tuned in real time.
In summary, the above three white LED packaging methods for emitting white light have at least one of the following drawbacks. First, the emitted white light is provided with fixed color temperature. Second, the color temperature of the emitted white light can not be tuned in real time. Third, the light distribution of the mixing white light is non-uniform. Thus, the color temperature of the white light cannot be tuned for lighting applications, and according to the user preference or requirements of lighting applications